1. Field of Art
The disclosure generally relates to the field of electronic paper displays. More particularly, the invention relates to updating electronic paper displays.
2. Description of the Related Art
Several technologies have been introduced recently that provide some of the properties of paper in a display that can be updated electronically. Some of the desirable properties of paper that this type of display tries to achieve include: low power consumption, flexibility, wide viewing angle, low cost, light weight, high resolution, high contrast, and readability indoors and outdoors. Because these displays attempt to mimic the characteristics of paper, they are referred to as Electronic Paper Displays (EPDs) in this application. Other names for this type of display include: paper-like displays, zero power displays, e-paper and bi-stable displays.
A comparison of EPDs to Cathode Ray Tube (CRT) displays or Liquid Crystal Displays (LCDs) reveals that in general, EPDs require much less power and have higher spatial resolution, but have the disadvantages of lower update rates, less accurate gray level control, and lower color resolution. Many electronic paper displays are currently only grayscale devices. Color devices are becoming available often through the addition of a color filter, which tends to reduce the spatial resolution and the contrast.
Electronic Paper Displays are typically reflective rather than transmissive. Thus they are able to use ambient light rather than requiring a lighting source in the device. This allows EPDs to maintain an image without using power. They are sometimes referred to as “bi-stable” because black or white pixels can be displayed continuously, and power is only needed when changing from one state to another. However, many EPD devices are stable at multiple states and thus support multiple gray levels without power consumption.
The low power usage of EPDs makes them especially useful for mobile devices where battery power is at a premium. Electronic books are a common application for EPDs in part because the slow update rate is similar to the time required to turn a page, and therefore is acceptable to users. EPDs have similar characteristics to paper, which also makes electronic books a common application.
While electronic paper displays have many benefits there are disadvantages. One problem, in particular, is known as ghosting. Ghosting refers to the visibility of previously displayed images in a new or subsequent image. An old image can persist even after the display is updated to show a new image, either as a faint positive (normal) image or as a faint negative image (where dark regions in the previous image appear as slightly lighter regions in the current image). This effect is referred to as “ghosting” because a faint impression of the previous image is still visible. The ghosting effect can be particularly distracting with text images because text from a previous image may actually be readable in the current image. A human reader faced with “ghosting” artifacts has a natural tendency to try to decode meaning making displays with ghosting very difficult to read.
One method for reducing error, therefore reducing ghosting, is to apply enough voltage over a long period of time to saturate the pixels to either pure black or pure white before bringing the pixels to their desired reflectance. FIG. 1 illustrates a prior art technique for updating an electronic paper display. Here, display control signals (waveforms) are used that do not bring each pixel to the desired final value immediately. The original image 110 is a large letter ‘X’ rendered in black on a white background. First, all the pixels are moved toward the white state as shown by the second image 112, then all the pixels are moved toward the black state as shown in a third image 114, then all the pixels are again moved toward the white state as shown in the fourth image 116, and finally all the pixels are moved toward their values for the next desired image as shown in the resulting image 118. Here, the next desired image is a large letter ‘O’ in black on a white background. Because of all the intermediate steps this process takes much longer than the direct update. However, moving the pixels toward white and black states tends to remove some, but not all, of the ghosting artifacts.
Setting pixels to white or black values helps to align the optical state because all pixels will tend to saturate at the same point regardless of the initial state. Some prior art ghost reduction methods drive the pixels with more power than should be required in theory to reach the black state or white state. The extra power insures that regardless of the previous state a fully saturated state is obtained. In some cases, long term frequent over-saturation of the pixels may lead to some change in the physical media, which may make it less controllable.
One of the reasons that the prior art ghosting reduction techniques are objectionable is that the artifacts in the current image are meaningful portions of a previous image. This is especially problematic when the content of both the desired and current image is text. In this case, letters or words from a previous image are especially noticeable in the blank areas of the current image. For a human reader, there is a natural tendency to try to read this ghosted text, and this interferes with the comprehension of the current image. Prior art ghosting reduction techniques attempt to reduce these artifacts by minimizing the difference between two pixels that are supposed to have the same value in the final image.
Another reason that the prior art technique described above is objectionable is because it produces a flashing appearance as the images change from one image to the next. The flashing can be quite obtrusive to an observer and gives a “slide show” presentation quality to the image change.
It would therefore be highly desirable to have a method for updating an electronic paper display where the error in the subsequent image is reduced, thus displaying less “ghosting” artifacts when a new image is updated on the display screen, without the undesirable and interruptive effect when transitioning from one image to the next.